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Requirements

Prerequisite knowledge

A solid understanding of delivering video using Flash, including the ActionScript APIs for video delivery and playback. Also helpful to know the basics of HTTP, video delivery network protocols, and video encoding standards.

User level

Required products

Sample files

For the past few years, video has been one of the strongest trends on the web, mainly pushed by Adobe Flash Player. H.264 and full-screen support introduced in Flash Player 9 in 2007 really changed the game by making immersive HD video experiences on the web possible. Recent implementations of Flash Player on mobile devices have brought new ideas to the Flash Player team at Adobe for how to display video in Flash and always improve the user experience. Stage video is the result of those efforts.

The traditional method for rendering video in Flash Player uses the Video object. The Video object is treated the same as any other object on the stage, which gives developers an unprecedented amount of creative control. For example, video can be displayed on each face of a spinning cube, or multiple videos can be blended together with one another. The Video object is treated as any other DisplayObject. Figure 1 illustrates this idea.

Figure 1. Multiple videos can be blended together using the Video object.

To support that level of creative control, Flash Player must do a significant amount of processing for each video frame. Depending on the power of the underlying device, this increased processing may decrease the frame rate of the video, or it may increase the load that Flash Player places on the CPU.

A new way to render video

To mitigate the performance impact of rendering video in the Video object, Adobe has introduced stage video as a new way to render video. This approach takes full advantage of the underlying video hardware. The result is a much lower load on the CPU, which translates into higher frame rates on less powerful devices and also less memory usage. With stage video, the StageVideo object does not sit inside the Flash Player display list but sits behind the stage instead. Figure 2 illustrates this design.

Figure 2. The StageVideo object sits behind the Flash stage.

The performance benefits of stage video are especially pronounced for televisions, set-top boxes, and mobile devices. These devices do not have CPUs as powerful as desktop computers, but they do have very powerful video decoders capable of rendering high-quality video content with very little CPU usage. However, even on the desktop, stage video will dramatically change video performance in Flash Player.

As a developer, you must understand that stage video is the second step of enhancements related to video GPU acceleration in Flash Player. The first step is encoding the video so as to take best advantage of the hardware acceleration available on the target platform. Hence, to get the best experience possible, you will need to have the two steps enabled. The H.264 codec is stage video's best friend; using this will ensure you get full GPU acceleration from video decoding to rendering. With this approach, no read-back (sending the data from the GPU to the CPU) is required to composite the video frames in the display list anymore. The YUV 4:2:0 formatted video frames are converted to RGB through a GPU shader (DirectX9 or OpenGL) and blitted onscreen. As a result, you will see higher pixel fidelity and some reduction in CPU and memory usage.

Limitations

With stage video, the video is rendered onto a flash.media.StageVideo object rather than a Video object. The StageVideo object is always shown in a window-aligned rectangular region of the screen. Other graphics may be layered on top of the StageVideo object, but it is not possible to layer objects behind the video. Because StageVideo does not sit in the traditional display list but is composited through the GPU, the following features are not available when the StageVideo object is used:

The StageVideo object cannot be rotated. Only orthogonal rotations (rotations in ninety-degree increments) are possible.

The StageVideo object may not have a colorTransform or 3D transformations transform applied to it. It may not have a matrix transform applied that skews the video.

The StageVideo object cannot have an alpha channel, blendMode, filter, mask, or scale9Grid applied to it.

The video data cannot be copied into a BitmapData object (BitmapData.draw).

The video cannot be bitmap-cached.

The video data must not be embedded in the SWF file. StageVideo can be used only with videos originating from a NetStream object.

Depending on the underlying hardware, some color spaces may not be supported. In such cases, Flash Player will choose a substitute color space. The new StageVideo ActionScript API provides a means to query the color space that is being used.

Depending on the platform, the number of videos you are allowed to display in the video plane is limited. On most mobile systems, only one video is allowed at any time globally. That means that if you have several instances of SWF files showing at the same time, only the first SWF file will be able to display video in a hardware-accelerated fashion.

To ensure compatibility between Flash Player on desktop and on TV devices, set wmode to direct.

Avoid layering wmode="transparent" SWFs on top of each other. Platforms such as Google TV do not support wmode="transparent". This means that all SWF instances are supported when wmode="window" regardless of the <embed> tag parameters.

In practice, none of the above restrictions will affect the most common use case, which is a video player application. In cases where these restrictions are acceptable, developers are strongly encouraged to use the StageVideo object. Stage video is supported on the Google TV platform, on all AIR for TV platforms, and it will soon be included on all platforms that support Flash.

Requirements

To make sure stage video will be available, you must use wmode="direct" at all times. This mode is highly recommended for video playback. It uses Direct3D on Windows and OpenGL on Mac OS and Linux to actually perform video frame compositing directly through the GPU. The limitation is that, in this mode, Flash Player runs in its own context and having overlapping HTML content on top of the player is not possible, for instance. If you decide to go with any other mode, such as wmode="window", wmode="opaque", or wmode="transparent", you highly decrease the chances (see note) to have stage video available. As a result, for consistency, it is highly recommended to use wmode="direct" at all times.

Note: Some browsers such as Safari 4 (or later) or Internet Explorer 9, where libraries like CoreAnimation (MacOS 10.6) or IE9 GPU APIs (Windows Vista/7) can be leveraged, allow Flash Player to do compositing through the GPU in the context of the browser as with wmode="direct". As a result, this allows stage video to be enabled regardless of the value of the wmode parameter. But again, for consistency across browsers, just try to use wmode="direct" as much as possible.

Now that you have learned the concept and limitations of stage video, see what it looks like in terms of ActionScript implementation.

The Stage Video API

Beginning with Flash Player 10.2, there is a new class called StageVideo that represents a single video display instance in the hardware video plane. StageVideo objects are created by Flash Player and cannot be instantiated on their own. StageVideo objects can be accessed from the stageVideos vector available on the Stage object:

The length of the stage.stageVideos vector will vary, depending on platform and hardware availability, when the stageVideos property is accessed. The maximum number of StageVideo objects you can expect is eight. Hence, if multiple StageVideo objects are required in your application, this is completely possible on the desktop. On mobile platforms, only one StageVideo object will be available, so you will have to take this into account. Also, the length of the vector will sometimes be zero. Rather than polling the length of the stageVideos vector manually, to implement stage video correctly, you should always listen to the StageVideoAvailabilityEvent.STAGE_VIDEO_AVAILABILITY event. This will inform you about stage video ability.

At any time, you can listen to such an event and wait for the event to be dispatched to react appropriately:

It will be dispatched once, right after registering your event handler, and whenever the stage video availability changes over time. For more details about this behavior, check the Gaining or losing stage video section.

In the onStageVideoHandler, rely on the availability property available in the StageVideoAvailability event object:

StageVideoAvailability.AVAILABLE: Stage video is available; at least one StageVideo object is waiting in the stageVideos vector.

StageVideoAvailability.UNAVAILABLE: Stage video is not available; the stageVideos vector is empty.

Usually, once informed about the availability, you can decide what to do. If stage video is available, a StageVideo object can be retrieved from the stage.stageVideos vector. If stage video is not available, you will rely on a classical Video object you have already created and use it as a fallback:

You may have noticed that we also listen to a new event called StageVideoEvent.RENDER_STATE on the StageVideo object. Note that this new event is also available on the classical Video object through a similar event called VideoEvent.RENDER_STATE, and informs us on how the video is being rendered:

In Flash Player 10.1, it was not possible to know if the video frames were decoded and composited through the GPU or not. This new event added to the StageVideo and Video objects is fixing this limitation, beginning with Flash Player 10.2. The status property can have the following values, available as constants on the StageVideoEvent or VideoEvent classes:

VideoStatus.ACCELERATED: The video is being decoded and composited through the GPU.

VideoStatus.SOFTWARE: The video is being decoded though software and composited by the GPU, if dispatched by StageVideo, or through software, if dispatched by Video.

VideoStatus.UNAVAILABLE: The video hardware has stopped decoding and compositing the video.

The StageVideoEvent is dispatched as soon as the NetStream is attached to the StageVideo object, or VideoEvent for the Video object. For instance, if the status property from those events refers to VideoStatus.SOFTWARE, this informs you that the video is being decoded through software. Then you can try switching to another video stream—such as H.264 or a stream with different dimensions—to give better chances to have it decoded on the GPU. If hardware decoding is suddenly unavailable, you can fall back to a non-GPU accelerated codec to force software decoding. Such an event can also be used for debugging or logging to track the user experience.

As you can see, you can also use this event to handle video resize, as this new event also tells you when the video size can be retrieved from the Video or StageVideo objects and final width and height calculated depending on the constraints.

This code computes the best size in the space available while preserving the video ratio:

Remember that StageVideo is not a DisplayObject, so it does not implement all the expected properties that you'd use to position and scale a DisplayObject in Flash. The previous code used the viewPort property rather than expected properties like width and height to specify the video dimensions onscreen.

The StageVideo object exposes the following properties:

colorSpaces:Vector.<String>: Color spaces available on the underlying hardware.

depth:int: Depth of the StageVideo object. This property allows you to handle z-ordering among multiple StageVideo objects.

pan:Point: Panning (similar to x and y); a Point object must be specified. By default, the value of pan is (0,0).

videoHeight:int: Native height of the video stream; a read-only property.

viewport:Rectangle: Visible surface (similar to width and height); a Rectangle object must be specified.

zoom:Point: Zooming factor; a Point object must be specified. By default, the value of zoom is (1,1).

Note that StageVideo instances are rendered in order. The first StageVideo object in the stageVideos vector is rendered first; the next ones are rendered on top of the previous ones. To change this, you can manuallychange the order by using the depth property:

sv.depth = 0;
sv2.depth = 1;

You may have noticed that the StageVideo object exposes a colorSpaces property returning information regarding color spaces handled by the current hardware. Now you'll discover now how this can be useful.

Working with color spaces

The colorSpaces property can be accessed on a StageVideo object and returns a Vector of Strings:

var colorSpace:Vector.<String> = stageVideo.colorSpaces;

Color space names are enumerated in the flash.media.VideoColorSpace class:

Note that the player will try to match the color space of the StageVideo to the color space of the video stream. On some machine configurations, if this matching cannot be satisfied, Flash Player will then try to find the closest match possible.

Keep in mind that some video containers may embed information regarding the native color space of the video stream. As a result, Flash Player must respect that. A common case is H.264, in which video streams are usually encoded in "BT.709"—while the colorSpaces property might return "BT.601", meaning the underlying OS/graphics hardware did not have the capability to render a surface in "BT.709" color space. If this happens, you can either revert back to using software compositing (by using a classical Video object) or accept the color mismatch. If "unknown" is returned, the platform does not offer a way to query the actual color space being used.

Gaining or losing stage video

Stage video may not be available when your SWF file is instantiated, but it may become available at some time later. You may ask, why? As I explained before, the wmode value you will choose when integrating your SWF file will determine the stage video consistency across browsers. In some scenarios, you may not have the ability to set wmode="direct"—for integration reasons in the HTML page, for instance. Also, in full-screen mode, as Flash Player does not run in the browser context, stage video will always be available regardless of the wmode value. But note that when leaving full-screen mode, stage video may then become unavailable again, placing your SWF file back in the browser context, impacted by the wmode parameter.

Note: Because of this behavior, in a video player leveraging the StageVideo API, you should always have a classical Video object to be used as a fallback.

As a result, you need to structure your video player in a way to react appropriately. Luckily, this is fairly easy. If stage video becomes available, just attach the NetStream object to the StageVideo object just retrieved. When it becomes unavailable, just attach the previously attached NetStream to the Video object used as a fallback.

Typically, you would modify your toggleStageVideo function as follows:

This code now handles properly situations where stage video becomes available or unavailable with fallback to a classical Video object. When attaching the NetStream back to a StageVideo object, as hardware resources must be allocated, you may expect a little latency between the moment when you attach the NetStream and the moment you see the pixels onscreen.

If you want to know more about how to leverage the StageVideo APIs, check the files attached to this article. It contains a simple video player illustrating all the different scenarios.

Scenarios

As you have seen throughout this article, different scenarios can be met when playing video in Flash Player. Here is a list of the different paths you can encounter when playing video with the Video or StageVideo objects:

Video object playing a non-accelerated codec: CPU will be used for decoding and compositing.

Video object playing a GPU accelerated codec (H.264): GPU will be used for decoding but CPU will still be used for compositing.

StageVideo object playing a non-accelerated codec: CPU will be used for decoding but GPU will be used for compositing.

StageVideo object playing a GPU accelerated codec (H.264): GPU will be used for decoding and compositing. CPU will not be used at all for the entire pipeline. This is the "direct path" scenario you want to reach for best performance. Note that on Mac OS, hardware decoding of the H.264 video stream is available starting only with Snow Leopard, Mac OS 10.6.3.

Where to go from here

Be sure to try out Flash Player 10.2, which introduces new features and enhancements, including a new video hardware acceleration model that enables dramatically enhanced video playback performance. Also visit the Stage Video topic page on the Adobe Developer Connection for links to demos.

To fully enjoy this new StageVideo API, Adobe has introduced two little improvements that were highly requested by the community:

Support for full-screen mode with multiple monitors: Full-screen content will remain in full-screen mode on secondary monitors, allowing users to watch full-screen content while working on another display.

Full-screen detection capability: The new allowFullScreen property, now available on the Stage object, enables developers to detect whetherthe current container/hosting page allows the player to go full-screen.

The StageVideo APIs will definitely change video performance in Flash. In some scenarios, stage video can decrease reduce processor usage by up to 85 percent. Even with the requirements to leverage it, make sure to use it as much as possible. This will allow developers to leverage complete hardware acceleration of the video-rendering pipeline, enabling best-in-class video playback performance.